CA15-3, a new tumor marker for breast cancer, was determined in various malignant diseases including breast cancer and various benign diseases, and its clinical significance and usefulness were studied. In 18 normal individuals, the value of CA15-3 was 8.9 +/- 3.3 U/ml (mean +/- SD). In primary breast cancer, the positivity was 20% for Stage I, 0% for Stages II and III and 100% for Stage IV. Of 17 cases of recurrent breast cancer, 13 (77%) were shown to be positive. The therapeutic effect and the value of CA15-3 were well correlated with each other. As for other malignant tumors, positive cases were observed in 50% of recurrent cancer of the stomach and in 14% of malignant tumors of the biliary system. All of these cases were terminal-stage cancers. The CEA value determined simultaneously showed a good correlation, r = 0.87 (p less than 0.01) with CA15-3 in malignant tumors other than breast cancer. In breast cancer, however, the correlation between the two was low, r = 0.18. These results suggest that CA15-3 is not necessarily useful in the diagnosis of primary breast cancer, but is useful as an indicator of the effect of therapy for recurrent breast cancer and for the prediction of recurrence.